Thermal treatment of aluminum and aluminum base alloys



Patented Sept. 7, 1937 PATENT OFFICE THERMAL TREATMEN ALUMINUM Philip T. Stroup,

T OF ALULIINUM AND BASE ALLOYS New Kensington,

Pa., assignor to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing.

9 Claims.

This invention relates to the thermal treatment of aluminum and aluminum base alloys. It is more particularly concerned with protecting such materials against a type of attack which may occur at elevated temperatures under certain conditions.

Some kind of thermal treatment is generally used during the fabrication of articles from aluminum and aluminum base alloys, such as preheating the metal before hot working, or applying a solution heat treatment to certain alloys to increase their strength and hardness. These treatments are frequently carried out in furnaces having air atmospheres, because such furnaces are convenient and economical to operate. It has been Observed that when aluminum and aluminum base alloys are exposed to sufficiently elevated temperatures for a considerable length of time in an air atmosphere, the metal is sometimes subject to a certain type of attack which differs from the surface oxidation that normally occurs with aluminum. The attack is characterized by a roughening of the metal surface in the initial stages, and in the later stages by such an objectionable blistering and permanent discoloration of the metal surface that the articles must usually be discarded, since it is generally not economical to reclaim them.

The blisters which appear under the above described conditions are readily distinguishable in shape and mode of occurrence from those blisters that are sometimes found on sheets of annealed metal. The latter type of blister results from the presence of gas in the metal during the annealing operation, and for this reason it is commonly referred to as a gas blister. The blisters which characterize the high temperature attack may even occur on metal that is substantially free from gas. The discoloration referred to hereinabove varies between a dull gray and a black, and it is further characterized by a fused appearance-dull on some alloys and glazed on others, although there is no microscopic evidence of any incipient fusion. This discoloration is easily distinguishable from the dull oxide film or tarnish that is normally found on aluminum and aluminum base alloys, and is of so permanent a nature that it is impractical to remove it by the ordinary commercial etching solutions. Because this type of attack differs from normal oxidation, and for the sake of convenience in referring to it, I shall'designate it hereinbelow as high temperature blistering.

An investigation of the instances where high temperature blistering has occurred disclosed the temperatures above about 800 beryllium can be employed in 'tilized chloride to produce a Application October I, 1936, Serial No. 103,536

constituents such as water vapor, ammonia, or

sulphur compounds. It has also been found that the presence of certain elements in the alloys, such as magnesium, especially when in combination with copper, nickel, silicon and/orzinc, tends to increase the susceptibility to this attack. In practically all cases, the blistering occurred at E, which is above the range usually employed for annealing the cold worked metal.

Although aluminum and aluminum base alloys do not always suffer from high temperature blistering, even at temperatures above 800 R, such blistering does sometimes occur, and because of its objectionable character, it becomes important to minimize or eliminate even an occasional occurrence. A principal object of my invention is to provide a method of inhibiting high temperaturev blistering without impairing any of the useful properties of the alloys subject thereto,

My invention is predicated upon the discovery that the addition of a very small quantity of beryllium to aluminum and aluminum base alloys, especially those alloys containing between 0.1 and 12 per cent magnesium, and the introduction of a small amount of a vaporous chloride into the furnace atmosphere in which the aforesaid metal is heated, will prevent high temperature blistering. The combination of beryllium in the metal and a chloride in the atmosphere appears to produce a unique protective film the alloy. Neither substance when used apart from the other inhibits blistering at elevated temperatures, but in combination they are effective for this purpose. No alloy constituent other than aluminum base alloys, so far as I am aware, to react with the volafilm which protects the underlying metal from high temperature blistering,

Only a very small amount of beryllium is needed to accomplish my purpose, and in my preferred practice amounts less than 0.025 per cent are usually employed, since such amounts are completely soluble in aluminum at temperatures above 800 F. It appears from my, investigation that only the beryllium in solid amounts greater than 0.025 per cent are probably not soluble in the solid metal at the temperature of heat treatment. However, when larger amounts, such as 0.08 or 0.1 per cent or more of beryllium are present, I obtain the same results as with a smaller amount, since the undissolved element does not interfere with the action of the dissolved portion. Since beryllium is an expensive alloying constituent,- I prefer not to use more 5 than about 0.1 per cent, which is sufllcient to produce the desired result. On the other hand, at least 0.001 per cent beryllium should be present in order to afford the desired protection.

The amount. of chloride in the atmosphere which is required in combination with the beryllium in the alloy to obtain the desired protection is very small. Concentrations from about 0.001 to 0.05 milligram of chloride per liter of furnace atmosphere are adequate for mypurposes, and is higher concentrations of chloride may tend to etch the metal. A concentration of 0.01 milligram per liter of, atmosphere has been found to be very effective under most circumstances. The amount and concentration of chlorides needed in any particular instance depend upon the composition of the alloy, the temperature to which the metal is heated, and the length of exposure to said temperature. In general, the more severe the conditions, the greater should bethe amount and a concentration of chloride. These factors may be easily determined by drawing a definite volume of the furnace atmosphere into an absorbing solution, such as potassium hydroxide, and determining the chloride content by common chemi- 80 cal methods. v

The source of the chloride in the atmosphere is not important. A convenient method is to place some solid chloride salt in the furnace and permit it to volatilize under the influence of the 85 heat, or to hydrolyze through reaction with moisture in the atmosphere. If preferred, liquid or gaseous chlorides may be employed. Such solid chlorides as aluminum chloride, ferric chloride, calcium chloride and magnesium chloride, and

40 fluid chlorides such as hydrogen chloride, carbon tetrachloride, chloroform, and trichlorethylene, are among those suitable for my purpose.

Aluminum base alloys are generallymore susceptible to high temperature blistering than aluminum, and of the aluminum base alloys those containing between 0.1 and 12 per cent magneslum show a greater tendency to blister. Other elements, such as copper, silicon and zinc, may also be present in the alloy in amounts of from so 0.1 to 14 per cent, and they appear to render the alloys even more susceptible to high temperature blistering, especially when magnesium is also present. The presence of nickel in amounts up to 5 per cent, and the presence of such elements as manganese, chromium, titanium, molybdenum, tungsten and the like in amounts less than 2 per cent, may also increase the susceptibility of aluminum base alloys containing magnesium to high temperature blistering. The magnesium may be present as the major alloy component aside from aluminum, or it may be a minor constituent as in the example given hereinbelow.

The high temperature blistering only becomes important where temperatures above about 65,800 F. are encountered. Thus this deleterious ieflect does not occur at the usual annealing tem- 'peratures of 650 to 700 F., but only at temperatures in excess of 800 F., such as are included in the temperature ranges normally employed for preheating ingots, billets, slabs, etc., prior to hot working them, and for solution heat treatment of aluminum base alloys. The temperatures used in these thermal treatments seldom, if ever, exceed 1100 F., and higher temperatures may generally "15 be disregarded for thatreason. However. my

' tained no chlorides.

method of protecting alloys herein described is eflective even at temperatures above 1100 F. The amount of beryllium in the alloy and chloride in the atmosphere that are needed under any particular set of conditions may be readily determined by a few preliminary tests.

The'fact that beryllium alone is ineffective in preventing high temperature blistering of alloys of the type herein described, when they are heated in an atmosphere containing no vaporous chlorides, is illustrated by the following tests. Two alloys were prepared, one consisting of aluminum, 4.2 per cent copper, 0.5 per cent manganese and 1.5 per cent magnesium, and the other having the same composition with the addition of 0.004

per cent beryllium. Specimens of these alloys in the form of'she'ets were heated for 4 hours at 930 F. in an ordinary air atmosphere which con- All the specimens were badly blistered and discolored, indicating that beryllium alone is ineffective in eliminating high temperature blistering.

The effect of a chloride in the furnace atmosphere, with and without the presence of beryllium in the alloy, is shown in the following tests. Specimens of the alloy containing no beryllium described in the preceding paragraph, and specimens from three alloys of the same composition with the addition of 0.001, 0.005 and 0.01 per cent beryllium, respectively, were heated for 12 hours at 930 F. in an atmosphere containing a chloride supplied by volatilizing aluminum chloride in the furnace chamber. The chloride concentration was about 0.01 mg. per liter of atmosphere. The alloy containing no beryllium was blistered and discolored, but the alloys containing this element were entirely free from such defects.

The beryllium may be added to the alloys in any suitable manner, as for example, by means of a rich aluminum-beryllium or aluminum-copper-beryllium alloy, or by the decomposition of a beryllium compound. Once the element has been added, no particular care need be exercised to prevent burning or a settling out of segregations in the melting pot or in castings.

The term aluminum as herein employed refers to metal of commercial purity containing the usual impurities, and the term aluminum base alloys refers to those alloys containing more than 50 per cent aluminum. Both aluminum and aluminum base 'alloys are comprehended within the term aluminous metal or material.

I claim:

1. In the art of thermally treating aluminous metal, the method of protecting said aluminous metal against high temperature blistering during thermal treatment, comprising incorporating beryllium as a constituent of said aluminous metal and introducing a vaporous chloride into the atmosphere surrounding the metal-during said treatment.

2. The method of protecting aluminous metal against high temperature blistering during thermal treatment, comprising incorporating less than 0.1 per cent beryllium in said aluminous metal and introducing a vaporous chloride into the atmosphere surrounding the metal during said treatment.

3. The method of thermally treating aluminous metal in an air atmosphere without high temperature blistering, comprising incorporating a relatively small amount of beryllium in said metal, introducing a vaporous chloride into the air atmosphere surrounding the metal, and heating said metal in said atmosphere.

. constituent of the alloy,

4. The method of thermally treating aluminum base alloys which contain from 0.1 to .12 percent magnesium without high temperature blistering, comprising incorporating beryllium as a constituent of said alloy, introducing a vaporous chloride in the atmosphere surrounding the metal, and heating said alloy in said atmosphere.

5. The method of thermally treating aluminum base alloys without high temperature blistering, comprising incorporating from 0.001 to 0.1 per cent beryllium in said alloy, supplying a vaporous chloride to the atmosphere surrounding the metal, and heating said metal in said atmosphere.

6. The method of thermally treating aluminum base alloys containing from 0.1 to 12 per cent magnesium in an air atmosphere without high temperature blistering, comprising incorporating between 0.001 and 0.1 per cent beryllium as a introducing a vaporous chloride in the atmosphere surrounding the metal, and heating said 'alloy in said atmosphere.

7. The method of thermally treating aluminous metal without high temperature blistering in an air atmos here, comprising incorporating beryllium in said metal as a constituent thereof, introducing a vaporous chloride into the atmosphere of the heating chamber in an amount sufi'icient to produce a concentration of 0.001 to 0.01 mg. per liter of air in said chamber, and heating said metal in said atmosphere.

8. In the art of thermally treating aluminum base alloys, the method of inhibiting high temperature blistering of said alloys in an air atmosphere, comprising incorporating from 0.001 to 0.1 in said alloy as a constituent thereof, and providing a gaseous atmosphere in the heat treating furnace containing between 0.001 and 0.01 mg. of a vaporous chloride per liter of atmosphere.

9. The method of inhibiting high temperature blistering in an air atmosphere of aluminum base alloys, comprising incorporating from 0.001 to 0.025 per cent beryllium in said alloy as a constituent thereof and introducing a vaporous chloride into said atmosphere in suflicient maintain a chloride concentration of 0.001 to 0.01 mg. per liter of atmosphere.

PHILIP T. STROUP.

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